In a part-time 4WD vehicle for switching two wheel drive and four wheel drive, for example, in the case of the two wheel drive by only rear wheels, front wheels and a front axle shaft are spline-engaged with each other, so that a front drive system including the front axle shaft, differential gears and propeller shafts operatively connected to the front wheels is rotated in accordance with the running of the vehicle, thereby reducing life of seal members and increasing noise and fuel cost. To solve these problems, in a free wheel hub clutch, the front wheels and the front axle shaft are separated from each other in the case of the two wheel drive to stop the idle running of the front drive system, and a hub clutch disposed in the front wheels is coupled as a locking state in the case of the four wheel drive.
A means for coupling and decoupling the hub clutch is constituted by a device mechanically operated manually or automatically, a device operated by fluid as an operating source, a device operated by vacuum or magnetic force, etc. For example, a device operated by fluid is disclosed in U.S. Pat. No. 4,293,061 in which a clutch mechanism can be freely coupled and decoupled by fluid pressure through a drive apparatus. A device operated by vacuum is disclosed in U.S. Pat. No. 4,627,512 in which an intake manifold of an engine is connected to an airtightly sealed chamber through a conduit, and the sealed chamber is controlled in a vacuum state or an atmosphere opening state, freely coupling and decoupling the clutch mechanism disposed within the sealed chamber. A device operated by an electromagnet is disclosed in Japanese Laid-Open Patent 59-37340 in which a clutch mechanism is freely coupled or decoupled by attracting force of an electromagnetic coil. Further, in Japanese Laid-Open Patent 62-143737, a motor is disposed in a wheel hub, and a rotary shaft of the motor is located in a place apart from the rotary center of the wheel when the vehicle is running so that the motor is not smoothly operated and the unbalance in rotation of the motor is caused by the difference in weight of the place where the motor is located.
However, when the switching operation of the conventional hub clutch is manually performed mechanically, an operator must move away from an operating room to operate the hub clutch. When the switching operation of the hub clutch is automatically performed mechanically, the coupling of the clutch is released once when the clutch is switched to move the vehicle forwards or backwards. Accordingly, the driver cannot rapidly drive the vehicle out of a road having a very bad condition. Further, noises tend to be generated by wearing of members at the operating time thereof, reducing the durability of the apparatus.
When fluid or vacuum is used as an operating source, it is necessary to dispose pipings maintained in a an airtightly sealed state so that it is necessary to reliably hold seal, causing problems with respect to maintenance in function. In particular, some problems are caused by wearing of members with respect to reliability of rotary portions thereof. When a magnetic force is used as an operative source, the magnetic force is rapidly reduced in accordance with distance so that it is necessary to dispose a strong electromagnet, increasing the size of the apparatus. Further, it is necessary to flow an electric current at any time to maintain at least a four wheel drive state, thereby increasing power consumption and causing problems in reliability of the apparatus.